This invention relates to the general field of photolithography with particular reference to correction of photoresist pattern bias.
Photolithography is one of the most important steps in semiconductor manufacturing. Almost all components making up the structure of modern semiconductors are defined using photolithography. With the recent increases in component integration, the semiconductor industry is generally capable of using pattern lines of 0.25 xcexcm or less. At this small size, the fidelity of the transfer of photolithography mask patterns to the photoresist plays a very important role. If the transfer of the mask pattern is not correct, it may introduce variances that exceed the tolerance of the critical dimension (CD) on the wafer.
The proximity effect causes a form of optical bias associated with photoresist images. For a given development time, whether or not a given area of a photoresist layer will be left or removed after the development process depends on the total amount of energy deposited in that area during its exposure to radiation.
The proximity effect can be compensated for, at least in part, by modifying any given feature in the opposite direction to the expected bias. Thus, a line that would otherwise come out too narrow can be drawn as wider than its true width, etc. The overall nature and scope of these corrections will vary with the particular photolithography process that is being used.
One solution to the proximity effect is the use of optical proximity correction (OPC). OPC compensates for the proximity effect by altering the mask image such that the resulting pattern matches the desired pattern of the non-altered mask image. OPC is commonly calculated by summing two Gaussian functions whose values depend on the CD defined by conventional design rules as well as on the wavelength of the exposing radiation. In general, the bias of lines that are part of a dense assemblage will be more positive than the bias of isolated (sparse) lines in optical mode. In the past, without the benefit of the current invention, there have been many attempts in mask designs to compensate for CD bias. However, none of those methods offer the simplicity of application provided by the present invention.
Consequently, a method and apparatus are desired which eliminate the need for different photolithography masks to compensate for bias between sparse and dense mask patterns. The method and apparatus should also eliminate the need to use more complicated design and production of OPC masks. As explained in the following, the present invention provides a method and system that meet these criteria and solve other problems in the prior art.
The present invention eliminates the need to compensate for bias between isolated and dense lines. This is accomplished by reducing the intensity of the radiation exposing the isolated lines to within the range of the intensity of the radiation exposing the dense lines. By placing an attenuator between a photolithography mask and its projected image, the intensity of radiation passing through a sparse pattern of the photolithography mask is attenuated so that the intensity of the projected sparse pattern falls within the same range as the intensity of radiation that passes the dense pattern of the photolithography mask. In this way, the bias between dense patterns and sparse patterns caused by differing radiation intensities during exposure is eliminated. The attenuator has both a transparent region and an attenuating region. The attenuating region is designed to attenuate only the radiation projected from the sparse patterns of the photolithography mask. The attenuator has materials on it that can attenuate the density of passed radiation to normalize the intensity of both low spatial frequency and high spatial frequency images such that they fall within the same intensity range.